Allow multiple events to be processed in FakeEventHub

Currently, we are only allowing fake event hub to return a single input
event during tests.

That means, tests have to call 'loopOnce' for every event that is
enqueued to fake event hub.

The eventhub interface, however, does not have such a limitation.
EventHubInterface allows up to 'bufferSize' events to be returned at
each 'getEvents' call.

Therefore, adjust FakeEventHub to use this interface in the full
capacity.

Bug: 169866723
Test: atest inputflinger_tests
Change-Id: Icb4dd8fffea9bfef51861ab38ba529a814e7577f